Except that on S3 upload there will be only three attempts to execute Lambda, and then it will give up. I can easily imagine a simple outage that will lead to three errors in a row, and the S3 files goes unaccounted.
You're far better of sending out an SNS or an SQS triggered by an S3 object creation event.
You're far better of sending out an SNS or an SQS triggered by an S3 object creation event.